A combined dot density and size modulation system uses dispersed dot halftoning in conjunction with dot size modulation to produce a halftone image in which both the density and size of the dots are modulated to control overall gray level. The dot density and size modulation system offers advantages over pure dot density modulation systems or pure dot size modulation systems because it allows an extra degree of flexibility which can be used to increase the visual quality of the halftoned pattern and/or increase the robustness of the halftoning to printer artifacts and variations. An input pixel value is used to independently produce a dot density value and a dot size value. The dot density value and dot size values may be obtained from, e.g., look up tables that have been optimized for print quality and printer stability. Dispersed dot halftoning is used to provide a halftone value for the desired pixel location using the dot density value. The dispersed dot halftoning may be, e.g., tone dependent error diffusion. The halftone value and the dot size value for the pixel location is then used to generate a modulated code, e.g., a pulse width modulated code, to the printer. The modulated code may include both the pulse width of the desired dot for the pixel location as well as the justification, e.g. left, center, or right, for the pixel location. The dot density and size modulation system is particularly useful in modern electrophotographic printing systems that allow the printed dot size to be almost continuously varied through the specification of a pulse width modulation (PWM) code.
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20. An image forming system for halftoning an image, said system comprising:
an image forming device operative to:
receive an input pixel value for a pixel location within the image;
modulate the dot density of the image by controlling the dot density for the pixel location within the image using the input pixel value and performing dispersed dot halftoning to produce a dot position based on the dot density; and
modulate the dot size of printed dots to obtain a printed halftone image by controlling the dot size for the pixel location within an image using the input pixel value and performing dot size modulation based on the dot size and the dot position.
1. A method of halftoning an image, said method comprising:
inputting an input pixel value for a pixel location within said image;
modulating the dot density of said image; and
modulating the dot size of printed dots to obtain a printed halftone image;
wherein modulating the dot density of said image comprises controlling the dot density for said pixel location within said image using said input pixel value and performing dispersed dot halftoning to produce a dot position based on said dot density;
wherein said modulating the dot size of said image comprises controlling the dot size for said pixel location within an image using said input pixel value and performing dot size modulation based on said dot size and said dot position.
16. A method of optimizing a dot size look-up table and a dot density look-up table for a printing system that uses dot size modulation and dot density modulation, the method comprising:
printing at least one test page showing the combinations of dot sizes and dot densities;
measuring the output absorptance for each combination and the print distortion for each combination;
determining the print distortion at each output absorptance using the measured output absorptance and the measured print distortion for each combination;
calculating the optimized dot size look-up table using the print distortion at each output absorptance; and
calculating the optimized dot density look-up table using the print distortion at each output absorptance.
2. The method of
3. The method of
4. The method of
5. The method of
6. The method of
7. The method of
8. The method of
9. The method of
generating a dot density value based on said input pixel value, said dot density value being used in said tone dependent error diffusion;
said tone dependent error diffusion, comprising:
combining said dot density value with at least one previous error value to produce a modified pixel value;
comparing said modified pixel value with a threshold value to produce a halftone value for said pixel location; and
using said halftone value for said pixel location to produce an error value that is diffused to at least one subsequently processed pixel.
10. The method of
11. The method of
12. The method of
13. The method of
15. The method of
17. The method of
inverting the output absorptance for each combination to compute the value of the dot density required to produce each output absorptance; and
using the inverted output absorptance and the print distortion for each combination to determine the print distortion as a function of output absorptance.
18. The method of
19. The method of
21. The image forming system of
22. The image forming system of
23. The image forming system of
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This application is related to application Ser. No. 09/648,531 which is incorporated herein by reference in its entirety.
Appendix A1, which is part of the present disclosure, is an appendix consisting of 8 pages. Appendix A1 lists source code written in C programming language of an illustrative embodiment of the present invention using an eight-bit code to control a printer system having a pulse width modulated laser capability for each printed pixel. Appendix A2, which is part of the present disclosure, is an appendix consisting of 13 pages. Appendix A2 lists the particular coefficients used in the Tone-Dependent Error Diffusion process for the eight-bit code used in accordance with an embodiment of the present invention. Appendix B1, which is part of the present disclosure, is an appendix consisting of 14 pages. Appendix B1 lists source code written in C programming language of an illustrative embodiment of the present invention using a two-bit code to control a printer system having a pulse width modulated laser capability for each printed pixel. Appendix B2, which is part of the present disclosure, is an appendix consisting of 2 pages. Appendix B2 lists source code written in C programming language that is used in conjunction with Appendix B1 for a two-bit code used to control a printer system having a pulse width modulated laser capability for each printed pixel. Appendix B3, which is part of the present disclosure, is an appendix consisting of 22 pages. Appendix B3 lists the particular coefficients used in the Tone-Dependent Error Diffusion process for the two-bit code used in accordance with an embodiment of the present invention.
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the patent and trademark office patent file or records, but otherwise reserves all copyright rights whatsoever.
The present invention relates to halftoning, and in particular to a halftoning method that combines dot size modulation and dot spacing modulation to control the overall gray level in an image forming or printing device.
Continuous-tone images, such as charts, drawings, and pictures, may be represented as a two-dimensional matrix of picture elements (pixels). The spatial resolution and intensity level for each pixel are chosen to correspond to the particular output device used. Typically, digital halftoning is used to transform a continuous-tone image into the desired matrix of pixels.
Conventional methods for digital halftoning generally fall into two categories: clustered dot and dispersed dot. As is well known in the art, in clustered dot techniques, the size of the printed dot is varied to control the perceived gray level or, equivalently, density of the printed tone. These methods may be thought of as amplitude modulation (AM) halftoning techniques since the amplitude or size of the dots controls the printed gray level.
Dispersed dot halftoning methods control the printed gray level through the spacing or, equivalently, the frequency of dot placement. Dispersed dot halftoning may be thought of as a frequency modulation (FM) halftoning technique since the frequency or spacing of the dots controls the printed gray level. Dispersed dot halftoning methods are well known, and include, for example, error diffusion halftoning, screening and, most recently, iterative search based halftoning.
Typically, dispersed dot halftoning uses the smallest dots possible to print as it is undesirable to actually notice the dots in the printed image. However, a drawback of dispersed dot halftoning is that the printer must be capable of printing well formed isolated dots. For example, error diffusion is typically used only in inkjet type printers because they generally can print stable isolated dots. However, error diffusion is typically not used in commercial electrophotographic printers and copiers, such as laser printers, due to their instability in producing binary or multilevel halftones.
A halftoning system, in accordance with an embodiment of the present invention, uses dispersed dot halftoning in conjunction with dot size modulation to produce an image in which the density and size of the dots are modulated in conjunction to control the overall gray level in an image forming system. The dot density is modulated to control the spacing or frequency of the dot to be printed for a particular pixel location with respect to dots to be printed at preceding and subsequent pixel locations, while the dot size is modulated to control the size of the dot to be printed at the particular pixel location. The dot density and size modulation system may be used with an image forming or printing system, e.g., a computer and a printer. It should be understood that the image forming or printing system of the present invention is intended to include, but is not limited to, an electrophotographic printer or copier, an ink jet printer, a facsimile machine or any other device that may be used to print an image. The dot density and size modulation system is particularly useful in modern electrophotographic printing systems that allow the printed dot size to be almost continuously varied through the specification of a pulse width modulation (PWM) code.
In accordance with an embodiment of the present invention, the dot density is modulated by controlling the dot density for a pixel location using an input pixel value for that pixel location and by performing dispersed dot halftoning to produce a dot position based on the dot density. For example, a look up table may generate a dot density value based on the input pixel value, which is then used in the dispersed dot halftoning process. The dispersed dot halftoning process may be error diffusion, e.g., tone dependent error diffusion, dispersed dot screening, or iterative search based halftoning. Tone dependent error diffusion is particularly advantageous because it generates high quality dispersed dot patterns and is computationally efficient.
The dot size is modulated, in accordance with an embodiment of the present invention, by independently controlling the dot size for the pixel location using the input pixel value and performing dot size modulation based on both the dot size and the dot position. Thus, for example, a separate look up table may be used to generate the dot size value based on the input pixel value. Advantageously, the dot size control and dot density control may be precomputed, and in one embodiment are designed with respect to each other and the characteristics of the printing system to optimize the image while matching the printing device characteristics. The resulting dot size value, as well as the dot position from the dispersed dot halftoning process, are then used to generate a signal, e.g., a pulse width modulated code, that controls the printer system. The signal may be produced by another look up table and may include information regarding both the pulse width of the desired dot for the pixel location as well as the justification, e.g. left, center, or right, for the pixel location.
In accordance with an embodiment of the present invention, the dot size look-up table and dot density look-up table are designed with respect to each other and the characteristics of the printing system in a closed loop measurement process to optimize the quality of the image while matching the printing device characteristics.
The system, in accordance with the present invention, uses a combination of the dot size and dot density to produce the desired gray level, which offers advantages over pure dot density modulation systems or pure dot size modulation systems. The combination of dot density and dot size modulation allows an extra degree of flexibility, which can be used to increase the visual quality of the halftoned pattern and/or increase the robustness of the halftoning to printer artifacts and variations.
The dot density control 102 provides a dot density value that is received by the dispersed dot halftoning unit 108 to determine the positions of dots on the discrete printing grid. The dot size control 104 provides a dot size value that is received by the dot size modulation unit 110 along with the positions of the dots from the dispersed dot halftoning unit 108. The dot size modulation unit 110 generates an output signal indicating the modulated size and modulated density of the dot to be printed for the input pixel x(m,n) by the printing system.
Advantageously, both the dot density and dot size are varied independently based on attributes of the input image x(m, n). Nevertheless, neither the dot density control nor the dot size control independently control the printed tone. Instead, the printed tone is controlled through a combination of the two. The dot density and size are designed with respect to each other and the printing system to optimize the printed image while matching the device characteristics of the printing system. Consequently, tone correction may be incorporated directly into the combined density/size control.
As shown in
Typically, the quantity varied in dispersed dot halftoning is the dot spacing, which is specified using the number of dots per unit area or dot density. It should be understood, however, that for some dispersed dot halftoning methods the output of the dot density control may be only approximately equal to the true dot density. For more information regarding dispersed dot halftoning in general, see for example, J. Mulligan and A. Ahumada, Jr., “Principled Halftoning Based on Models of Human Vision,” Proc. SPIE/IS&T's symp., electr, imag. sci. and tech., vol. 1666, pp. 109–121, San Jose, Calif., February, 1992; and T. Pappas, and D. Neuhoff, “Least-Squares Model-Based Halftoning,” Proc. SPIE/IS&T's symp., electr, imag. sci. and tech., vol. 1666, pp. 165–176, San Jose, Calif., February, 1992, which are incorporated herein by reference.
The input pixel value is placed in a weight LUT 146, which may be, e.g., a 3×129×8 bit LUT, and which produces three different weights for the error diffusion filter 142. The input pixel value is also placed in a threshold weight LUT 148, which may be, e.g., a 2×129×8 bit LUT, and which produces two different thresholds for the threshold matrix 144. In addition, the pixel location (m,n) is received by a direct binary search screen (DBS) 150, the output of which is multiplied by the resulting output of threshold weight LUT 148, which is then summed with the other resulting output of threshold weight LUT 148. The summed result is then provided to the threshold matrix 144.
The error diffusion filter 142 provides an error value from previously processed pixels that is summed with the input pixel value X(m,n), to produce a modified pixel value u(m,n). The modified pixel value u(m,n) is received by the threshold matrix 144 and compared to at least one threshold level to produce the output halftone value g(m,n). A quantizer error value d(m,n) is produced as the difference between the output halftone value g(m,n) and the modified pixel value u(m,n). The quantizer error d(m,n) is received by the error diffusion filter 142, and is diffused to neighboring, subsequently processed pixel locations. See U.S. patent Ser. No. 09/307,003, entitled “Tone Dependent Error Diffusion” to Pingshan Li and Jan P. Allebach, filed May 7, 1999, which is incorporated herein by reference for a discussion of the implementation of the TDED system 140. As one of ordinary skill in the art understands, a TDED system may be optimized for particular printing systems and printing characteristics by altering the particular process coefficient. For the particular coefficients used in accordance with an embodiment of the present invention, see Appendix A2.
As shown in
The PWM system 130 produces a signal, e.g., an eight-bit PWM code, that controls the size of the printed dot, for example, produced by a laser in an electrophotographic printer. As is well understood in the art, in an electrophotographic printer, the toner is only deposited when the laser is on. Thus, each six-bit PWM code controls the width of a single pixel. The output signal from the PWM system 130 may also include two bits of information regarding the justification of the dot, i.e., whether the dot is left, center or right justified within the pixel.
The source code in Appendix A1 written in C programming language is one example of an implementation of the present invention using an eight-bit PWM code to control a printer system having a pulse width modulated laser capability for each printed pixel.
In
XPWM=DotSizeLUT(left pixel)>>1+RightHandJustification; and equ. 1
OPWM=(DotSizeLUT(right pixel)+1)>>1+LeftHandJustification. equ. 2
If a dot is not fired, then XPWM=OPWM=off. Here the values “left pixel” and “right pixel” denote the values of the continuous tone image to be printed at the corresponding left and right locations of the pixel pair.
TABLE 1
1st Bit
2nd Bit
Value
Justification
1
1
0xc0
Right
0
1
0x40
Left
0
0
0x00
Center
1
0
0x80
Undefined
. Then, for each row of pixels, the area of intersection between the row and the ideal dot is calculated. To do this, let k index the rows of the image, and let k=0 for the row containing the pixel (m, n). Then the region of intersection is defined by
Sk={(x,y):x2+y2<r and |x−k|<1/2} equ.3
where x and y are the distance from the center of the pixel (m, n). The area of intersection is then βk=Area{Sk}. If βk>1, then nk=2└(βk−1)/2┘+1 pixels in the kth row are fully turned on. For the remaining fractional area γk=βk−nk, two pixels on either side of the fully turned on pixels are turned on, each with area γk/2. The group of pixels are justified so that the dot is more compact, i.e., the left side pixel is right justified and the right side pixel is left justified so that the dot is formed by one pulse. If βk<1, we just turn on one pixel of area βk with center justification. This is illustrated in
When the dot area is greater than 1, i.e. θ is greater than 1, dot overlap may occur. In this case, the pulse widths are added, and only the first pulse justification is retained. More specifically, suppose there are N dots indexed by i=1, . . . , N that correspond to the same pixel location. Let bi be the justification part (the two most significant bits) of the PWM code, and let ai be the pulse width part (the remaining six bits) of an eight-bit PWM code. Then the PWM code for a pixel is determined by
The value of the pulse width is clipped if it exceeds the maximum value of 63. The effects of this clipping on the tone curve are removed when the dot density control LUT 122 (
In some cases, 63 pulse width levels of PWM may be available or less than 63 levels of pulse width modulation may be available, which may cause contouring artifacts in the printed images. In either case, contouring happens because a change of one level in the PWM code is substantial enough to cause a visible difference in gray level. To eliminate this problem, the pulse width codes may be dithered. The dithering may be done using random numbers, a white noise threshold mask, a dispersed dot threshold mask (such as the DBS screen 150 shown in
The dot density and dot size controls are selected to obtain the desired gray level, e.g. tone curve, for each input value x(m, n). Advantageously, the dot size control and dot density control may be precomputed, and in one embodiment are designed with respect to each other and the characteristics of the printing system in a closed loop measurement process to optimize the quality of the image while matching the printing device characteristics. Because the tone curve is controlled through the combination of the dot density control and dot size control there is an additional degree of freedom that may be used to optimize a variety of printing attributes including print quality and/or print stability. In other words, it is possible to achieve most desired tone curves by either varying the size of the dots or the density of the dots. Therefore, a particular choice of a dot size necessitates a specific dot density to achieve the desired tone curve. The object is then to select the dot size, at each desired output density, that produces the “best” printed output. The dot density LUT 122 and dot size LUT 124 are designed by experimentally measuring the quality of the printed halftones at each combination of dot size and dot density, and then choosing the best combination of dot size and density for each desired output gray level. Because the dot density control and dot size control are designed by directly measuring the print quality, the resulting design accounts for attributes of both the printer and the halftoning system used.
To measure the dot distortion Dθ(n) for each dot size θ and dot density n, a frequency weighted total squared error is used as the metric, where the frequency weighting is chosen to approximate the human visual system (HVS) response. The HVS model is a linear shift-invariant low pass filter. The frequency response of this filter is given by
where L is the average luminance in cd/m2, a=131.6, b=0.3188, c=0.525 and d=3.91.
Let f(m, n) denote the measured gray level measured in units of gamma corrected absorptance so that I(m,n)=(1−f(m, n))γ where I(m,n) is the normalized absorptance and γ=2.2. Then let
be the average absorptance level. The windowed error is computed as:
e(m,n)=(f(m,n)−g)w(m,n)(4) equ. 7
where w(m, n) is a Hamming window. The discrete Fourier transform (DFT) of e(m,n) is calculated to form E(k, l), and the print distortion metric is then given by
where H(k, l) is the appropriately sampled version of H(u, v).
Once the tone and distortion curves are determined for each dot size, e.g., θ=0.3 to 1.4, these curves are used to determine the distortion at each gray level (block 186). Each tone curve is inverted to compute the value of the dot density required to produce each gray level.
The inverse tone curves are used to determine the print distortion as a function of gray level.
Dθ,g=Dθ(nθ(g)) equ. 10
With these measurements, the optimized dot size LUT 124 and dot density LUT 122 can be calculated (blocks 188 and 190). These two look-up tables denoted by θi and ni, respectively, where i ranges from 0 to 255. The look-up table functions, gi and θi may be optimized so that both are smooth and achieve nearly optimal print quality at each desired absorptance level, by minimizing a cost function shown below as a function of the look-up tables θi.
Here σ is a parameter that controls smoothness of the result by forcing the values of the parameters θi to only change in small amounts. Because the print distortion function is only measured for discrete values of the dot size θ, the function must be interpolated using e.g., cubic spline interpolation, for missing values. Coordinate decent optimization is used to minimize the cost function; however, other optimization methods can be used and may be desirable, particularly ones that are robust to local minima in the cost functional.
Once the look-up table θi is determined, the corresponding dot density LUT 122 ni may be determined by using the inverse tone curves, nθ(g) shown in graph 220 in
ni=nθ
Again, because nθ(i) is not measured for each value of dot size θ, the intermediate values may be interpolated using, e.g., cubic spline interpolation.
While graphs 240 and 250 may be used as the dot size LUT 124 and dot density LUT 122 in the AM/FM halftoning system 120, in practice, the best results may require some manual adjustment of these curves. For example, the dot density curve in graph 250 is not monotone. While the resulting gray levels of the AM/FM halftoning system 120 are still monotone due to the increasing dot size at those gray levels, in practice, it may be more desirable to constrain the solution to have monotone increasing dot density.
It should be understood that variations of the present AM/FM halftoning system are possible. For example,
The source code in Appendices B1 and B2 written in C programming language are an example of an implementation of the present invention using a two-bit PWM code to control a printer system having a pulse width modulated laser capability for each printed pixel. Appendix B3 lists the particular coefficients used in the Tone-Dependent Error Diffusion process for the two-bit code used in conjunction with Appendices B1 and B2.
In one embodiment, the look-up tables and method for controlling the error diffusion process are provided on a computer readable medium, such as a microdiskette or floppy diskette as a printer driver. This printer driver is then installed into the computer so that the program is installed in the computer's RAM. Such a program may be also installed in the printer and, in one embodiment, installed in firmware within the printer. All logic functions may be implemented in hardware or software. Thus, for example, the image forming system may include a computer that is coupled to the printing device, where a computer program executed by the computer includes instructions for implementing the functions of the present invention. The computer may be, e.g., a host computer, microprocessor or any other appropriate device. If hardware is used, the various table values would be available to the circuitry implementing the halftone method via bus lines. The method may also be carried out by an ASIC, which controls the timing and transfer of data to the various logic devices and look-up tables as well as to and from the image map, as would be understood by those skilled in the art after reading this disclosure.
While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.
He, Zhen, Lin, Qian, Bouman, Charles A.
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Jan 31 2003 | Hewlett-Packard Company | HEWLETT-PACKARD DEVELOPMENT COMPANY, L P | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 026945 | /0699 |
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